A pulsive noise such as ignition noise inevitable in an audio device such as a car radio causes wave degradation or distortion in an entered audio signal and results in reproduced sound which offends human ears.
A prior art proposal of preventing this is the use of a signal compensation circuit as shown in FIG. 6, wherein reference numeral IN1 and OUT denote input and output terminals for inputting and outputting audio signals, IN2 designates a detection input terminal for receiving pulsive noises, 1 refers to a pulsive noise detecting circuit, and 2 to a signal compensation circuit including a phase shifting circuit 3, switches 4 and 5, a waveform compensation circuit 6 and a capacitor C.
An input signal S1 entered in the terminal IN1 is phase shifted into a signal S2 by the phase shifter 3 as shown in FIG. 7, thereafter processed by the waveform compensation circuit 6, and outputted from the terminal OUT.
When a pulsive noise is detected by the pulsive noise detecting circuit 1 via the terminal IN2, the detector 1 turns off the switches 4 and 5 for a predetermined duration TSW so as to block the input signal transmission to the waveform compensating circuit 6.
Referring to FIG. 7, if a pulsive noise N is produced in the period between times t1 and t2, the pulsive noise N is mixed up with the input signals S1 and S2 and distorts their waveforms.
Therefore, by detecting the pulsive noise N by the detector 1 and setting the switching time (off time) TSW equal to the duration between t1 and t2, the input signal transmission is stopped completely while the pulsive noise exists.
If the delay time of the phase shifter 3 for the input signal S1 is equal to the duration TSW, the level Q' of the signal S2 at the time t2 is known from the level Q of the signal S1 at the switching starting time t1, so as to establish a so-called "pre-holding".
Therefore, if the waveform compensation circuit 6 is activated to shape the waveform of the signal S2 in a continuous form between the levels P and Q' thereof, referring to the levels P and Q of the signals S1 and S2 at the time t1, the waveform of the phase-shifted signal S2 is compensated regardless that transmission of the signal S2 is stopped during the switching time TSW.
The prior art signal compensation circuit certainly operates effectively when the switching time TSW is relatively short i.e. one fourth or less, for example, of the wavelength of the received signal. However, if the switching time TSW is longer, the compensation effectiveness largely decreases. Therefore, the prior art signal compensation circuit cannot be used in a device for reception of high frequency signals.